1. Field of the Invention
The present invention relates in general to feed forward amplifier circuits. More specifically, the present invention relates to feed forward amplifier circuits with the ability to substantially null distortion produced by intermodulation products, thereby producing an output with a desired signal-to-interference ratio.
2. Description of the Prior Art
There are many applications in wireless communications that utilize feed forward amplifiers to amplify signals. However, a disadvantage of using feed forward amplifiers is that distortion is introduced into the signal as it is amplified, thereby resulting in an output signal that has a reduced signal-to-interference ratio. In the prior art there are many feed forward amplifiers that have been specifically designed to reduce the amount of distortion introduced during amplification. One such prior art approach is the use of a dual loop feed forward amplifier that actively nulls distortion. Such a prior art feed forward amplifier is shown in FIG. 1.
Referring to FIG. 1, a prior art dual loop feed forward amplifier 10 is shown. An RF signal is introduced into the signal input 12 of the dual loop feed forward amplifier 10. The purpose of the dual loop feed forward amplifier 10 is to amplify the RF signal with as little distortion as is possible. As the RF signal is received, it is split at coupler 13 between a main amplification line 14 and a first loop line 16. On the main amplification line 14, the RF signal passes through a series of amplifiers 18, wherein the RF signal is amplified. As the RF signal passes through the amplifiers 18, distortions are added to the RF signal. The RF signal with distortions is then again split at coupler 19, wherein the distorted signal is directed down to the first loop line 16 at coupler 21. The original RF signal traveling along the first loop line 16 passes through a delay line 20, wherein the original RF signal is delayed so as to be 180.degree. out of phase. As the out-of-phase signal is combined with the distorted signal at coupler 21, all elements of the two signals cancel except for the distortion elements that were added by the amplifiers 18. As such, at point P on the first loop line 16, only the distortion elements of the signal are present. The distortion elements of the signal then pass though an error amplifier 22, thereby producing an amplified distortion signal.
The amplified distortion signal is coupled to a first loop null point 24 by a coupler 26. The first loop null point 24 is coupled to a computer controller 28. The computer controller 28 is also coupled to a phase shifter circuit 30 positioned on the main amplification line 14 in between the RF input port 12 and the first amplifier 18. The computer controller 28 actively monitors the amplified distortion signal and adjusts the phase shifter circuit 30 within allowable bandwidth parameters in an attempt to minimize the amplified distortion signal.
The second loop of the dual loop feed forward amplifier 10 takes the amplified distortion signal and couples it back up to the main amplification line 14. The original amplified RF signal that travels along the main amplification line 14 passes through a second delay line 34 that changes the phase of the amplified RF signal so that it matches that of the amplified distortion signal. As a result, when the amplified distortion signal is superimposed over the original amplified RF signal, the error fundamentals carried by the original amplified RF signal are substantially canceled. This provides an amplified RF signal at the output port 36 with greatly reduced levels of distortion.
A phase shifter circuit 40 is also disposed within the second loop in between the error amplifier 22 and the coupler 41 that superimposes the distortion signal back onto the main amplification line 14. The phase shifter circuit 40 is selectively set at values intended to minimize the differences between the amplified distortion signal and the actual distortion created by the amplifiers 18 on the main amplification line 14. In order to "fine tune" the second loop, expensive and complex tuning techniques are commonly employed. Such prior art techniques require the use of advanced DSP software or clear channel hopping. When digital signal processing components or other additional components are used, the fine tuning of the feed forward amplifier 10 may lead to high costs for operation and maintenance. Furthermore, such prior art fine tuning techniques can produce undesirable changes in gain, distortion, bandwidth and/or impedance. It will also be understood that the filtering out of noise or intermodulation products is difficult since the intermodulation products are usually within the passband of the desired user signal. As a result, such filtering can result in unwanted amplitude or phase adjustments.
A need therefore exists for an improved dual loop feed forward amplifier that can efficiently and cost effectively reduce intermodulation products in an amplified RF signal, while producing a signal with a high signal-to-interference ratio without diminishing the amplitude level of the RF signal.